Fluid working machines and methods

ABSTRACT

A fluid working machine, operable to carry out a motoring cycle under at least some circumstances, comprises a controller ( 12 ) and a working chamber ( 2 ) of cyclically varying volume. The working chamber has a high pressure valve ( 18 ) associated therewith to control the connection of the working chamber to a high pressure manifold ( 20 ), and an electronically controllable primary low pressure valve ( 14 ) to control the connection of the working chamber to a low pressure manifold. The controller is operable to actively control at least the primary low pressure valve, in phased relationship to cycles of working chamber volume, to determine the net displacement of fluid by the working chamber on a cycle by cycle basis. The fluid working machine is adapted to release pressurised fluid from the working chamber prior to the opening of the primary low-pressure valve, during a motoring cycle. This facilitates the opening of the primary low pressure valve.

FIELD OF THE INVENTION

The invention relates to the field of fluid working machines,particularly fluid working machines which comprise at least one workingchamber of cyclically varying volume, in which the net displacement offluid through the or each working chamber is regulated by at least oneelectronically controllable valve, on a cycle by cycle basis, todetermine the net throughput of fluid through the or each workingchamber.

BACKGROUND TO THE INVENTION

Fluid working machines include fluid-driven and/or fluid-drivingmachines, such as pumps, motors, and machines which can function aseither a pump or as a motor in different operating modes.

When a fluid working machine operates as a pump, a low pressure manifoldtypically acts as a net source of fluid and a high pressure manifoldtypically acts as a net sink for fluid. When a fluid working machineoperates as a motor, a high pressure manifold typically acts as a netsource of fluid and a low pressure manifold typically acts as a net sinkfor fluid. Within this description and the appended claims, the terms“high pressure manifold” and “low pressure manifold” refer to manifoldswith higher and lower pressures relative to each other. The pressuredifference between the high and low pressure manifolds, and the absolutevalues of the pressure in the high and low pressure manifolds willdepend on the application. For example, the pressure difference may behigher in the case of a pump which is optimised for a high power pumpingapplication than in the case of a pump which is optimised to preciselydetermine the net displacement of fluid, for example, a pump fordispensing a metered amount of fluid (e.g. a liquid fuel), which mayhave only a minimal pressure difference between high and low pressuremanifolds. A fluid working machine may have more than one low pressuremanifold.

Although the invention will be illustrated with reference toapplications in which the fluid is a liquid, such as a generallyincompressible hydraulic liquid, the fluid could alternatively be a gas.

Fluid working machines are known which comprise a plurality of workingchambers of cyclically varying volume, in which the displacement offluid through the working chambers is regulated by electronicallycontrollable valves, on a cycle by cycle basis and in phasedrelationship to cycles of working chamber volume, to determine the netthroughput of fluid through the machine. For example, EP 0 361 927disclosed a method of controlling the net throughput of fluid through amulti-chamber pump by opening and/or closing electronically controllablepoppet valves, in phased relationship to cycles of working chambervolume, to regulate fluid communication between individual workingchambers of the pump and a low pressure manifold. As a result,individual chambers are selectable by a controller, on a cycle by cyclebasis, to either displace a predetermined fixed volume of fluid or toundergo an idle cycle with no net displacement of fluid, therebyenabling the net throughput of the pump to be matched dynamically todemand.

EP 0 494 236 developed this principle and included electronicallycontrollable poppet valves which regulate fluid communication betweenindividual working chambers and a high pressure manifold, therebyfacilitating the provision of a fluid working machine functioning aseither a pump or a motor in alternative operating modes. EP 1 537 333introduced the possibility of part cycles, allowing individual cycles ofindividual working chambers to displace any of a plurality of differentvolumes of fluid to better match demand.

Key factors which determine the performance of fluid working machines ofthis type include the performance characteristics of the electronicallycontrollable valves. These valves are typically electromagneticallyactuated poppet valves, although other valves types could conceivably beemployed. Relevant performance characteristics include the speed atwhich the electronically controllable valves open and close, thepressure difference against which they can open, their operationallifetime and the cross-section of the flow path through the valve whilstopen, which limits the throughput of fluid and influences the flowcharacteristics of fluid into and out of the working chambers.Accordingly, the electronically controllable valves are an expensive andperformance limiting component of such fluid working machines and itwould be desirable to reduce one or more of the demands made on theelectronically controllable valves.

A significant technical problem with fluid working machines of the typedescribed above relates to the opening of the low pressure valve, whichconnects a working chamber to a low pressure manifold, in a fluidworking motor (such as a fluid working machine which can function onlyas a motor, or a fluid working machine which can function either as amotor or a pump, in different operating modes). In a motoring cycle, ahigh pressure valve associated with the working chamber is closed, underthe active control of the controller, shortly before the end of theexpansion stroke. As the working chamber continues to expand, thepressure of the fluid trapped within the working chamber drops.Typically, the pressure of the fluid trapped within the working chamberwill need to drop to close to the low pressure manifold pressure beforethe low pressure valve can open. However, it can take a significantperiod of time for the pressure of the fluid trapped within the workingchamber to drop to a sufficiently low value, for several reasons.Firstly, the rate of change of working chamber volume decreases towardsthe end of the expansion stroke in most fluid working machines.Secondly, the variation in pressure of the fluid trapped within theworking chamber is not a linear function of the volume of the workingchamber, in the case of many commonly used hydraulic fluids.Furthermore, gases which are dissolved within the hydraulic fluid mayevaporate, which has the effect of reducing the expected rate ofdecrease of pressure within the working chamber. This delay can reducethe efficiency of the fluid working motor. Indeed, malfunctions canarise if the pressure within the working chamber does not drop to asufficiently low value to enable the opening of the low pressure valve,for example on start-up, or when operating in especially high or lowtemperature conditions.

Accordingly, some aspects of the invention aim to facilitate the openingof a low pressure valve, which regulates communication between theinterior of a working chamber and a low pressure manifold, during amotoring cycle of a fluid working machine.

Some embodiments of the invention addresses a further technical problem,which determines the specification of electronically controllable valvesfor a particular application, arises when fluid flows into a workingchamber of a pump from a low pressure manifold during an expansionstroke of a working chamber. The rate of fluid flow is limited by thecross-section and geometry of the flow path through the poppet valve andthe properties of the working fluid. Where the fluid flowing into theworking chamber is a liquid, it is subject to cavitation, whichincreases noise, reduces efficiency by requiring a pressure differenceacross the poppet valve, and leads to damage to the machine. A differentproblem applies during the contraction stoke of a working chamber in amotor, when fluid flows out to a low pressure manifold, where anincreased pressure drop causes inefficiency, and where the poppet valvemay be inadvertently closed causing possible damage to the valve andinadvertent pumping.

This problem has typically been solved by specifying largerelectronically controllable valves for higher throughput applications,or applications where superior fluid flow characteristics are required.However, larger electronically controllable valves are more expensiveand there can be a trade off in performance characteristics. Forexample, larger electronically controllable valves may open and closemore slowly than smaller valves or use more electrical power, forcingcompromises to be made. Some aspects of the invention also aim to reducethe build up of hot fluid that can occur in the crankcase in radialpiston pumps and/or motors.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided afluid working machine comprising a controller and a working chamber ofcyclically varying volume, the working chamber having a high pressurevalve associated therewith to control the connection of the workingchamber to a high pressure manifold, and an electronically controllableprimary low pressure valve to control the connection of the workingchamber to a low pressure manifold, the controller being operable toactively control at least the primary low pressure valve, in phasedrelationship to cycles of working chamber volume, to determine the netdisplacement of fluid by the working chamber on a cycle by cycle basis,the fluid working machine being operable to carry out a motoring cycleunder at least some circumstances, characterised in that the fluidworking machine is adapted to release pressurised fluid from the workingchamber prior to the opening of the primary low pressure valve, during amotoring cycle.

The resulting release of pressurised fluid preferably facilitates theopening of the primary low pressure valve. Preferably, the high pressurevalve is also electronically controllable and the at least one valveactively controlled by the controller typically also comprises the highpressure valve.

The fluid working machine may comprise depressurisation means which areoperable to release pressurised fluid from the working chamber prior tothe opening of the primary low pressure valve, during a motoring cycle,to facilitate the opening of the primary low pressure valve.

Preferably, the working chamber has a secondary low pressure portassociated therewith, which is openable and closable in phasedrelationship to the cycles of working chamber volume to releasepressurised fluid from the working chamber, for example, by connectingthe working chamber to a low pressure manifold, prior to the opening ofthe primary low pressure valve, during a motoring cycle, to reduce thepressure within the working chamber and thereby facilitate the openingof the primary low pressure valve.

Thus, by releasing pressurised fluid from the working chamber, prior tothe opening of the low pressure valve, during a motoring cycle, thepressure within the working chamber drops more quickly than wouldotherwise be the case, or to a lower value than would otherwise be thecase, facilitating the opening of the low pressure valve. Indeed, theopening of the secondary low pressure port may trigger the opening ofthe primary low pressure valve.

By releasing pressurised fluid from the working chamber prior to theopening of the primary low pressure valve we refer to releasingpressurised fluid from the working chamber prior to the opening of theprimary low pressure valve during a given motoring cycle. Typically, thepressurised fluid is released during the second half of an expansionstroke. Typically, the pressurised fluid is released after the highpressure valve closes. Typically, the pressurised fluid is releasedbetween the time when the high pressure valve closes and the time whenthe primary low pressure valve opens.

Preferably, the secondary low pressure port is openable and closable inphased relationship to the cycles of working chamber to releasepressurised fluid from the working chamber, by way of a mechanicalarrangement operatively linked to the expansion and contraction cyclesof the working chamber. Advantageously, a mechanical arrangement can beprovided which can open against a significant pressure differential,which substantially exceeds the pressure differential against which thelow pressure valve can open.

The timing of the opening and closing of the secondary low pressure portis selected depending on the intended application of the fluid workingmachine. For example, where the fluid working machine comprises arotatable shaft (e.g. in a rotary piston machine) and the fluid workingmachine is adapted so that the rotatable shaft rotates always orprimarily in one direction, the period of time between the opening ofthe secondary low pressure port and bottom dead centre may be differentto the period of time between bottom dead centre and the closing of thesecondary low pressure port. Where the fluid working machine operatesalways or primarily as a motor, the secondary low pressure port may beopened slightly before, at, or slightly after bottom dead centre, andthe secondary low pressure port may close significantly after bottomdead centre, and preferably at or after the point of maximum rate ofchange of working chamber volume intermediate bottom dead centre and topdead centre. Where the fluid working machine operates primarily as apump, the secondary low pressure port may close slightly before, or at,bottom dead centre.

Where the secondary low port associated with the working chamber isopenable and closable in phased relationship to the cycles of workingchamber volume to connect the working chamber to a low pressuremanifold, prior to the opening of the low pressure valve, during amotoring cycle, to release pressurised fluid and thereby reduce thepressure within the working chamber and facilitate the opening of thelow pressure valve, it may be that the secondary low pressure portremains open until at least the point in the subsequent contractionstroke where the rate of decrease of working chamber volume is greatest,to facilitate the flow of fluid out of the working chamber to one ormore low pressure manifolds. However, it may be that the secondary lowpressure port closes shortly after the low pressure valve has opened. Itmay be that the secondary low pressure port closes before the lowpressure valve opens.

The fluid working machine may comprise a rotatable shaft, such as acrankshaft. In this case, the opening and closing of the secondary lowpressure port may be operatively linked by a mechanical arrangement tothe angle of the rotatable shaft. Accordingly, the primary low pressurevalve may be openable on a cycle by cycle basis under the active controlof the controller, but the opening and closing of the secondary lowpressure port may not be variable on a cycle by cycle basis, and may befixedly phase locked to the expansion and contraction cycle of theworking chamber, e.g. by virtue of a mechanical arrangement operativelylinked to the angle of a rotatable shaft, where present. The secondarylow pressure port may comprise a mechanically actuated valve operated bya pushrod mechanically linked to the expansion and contraction cycles ofthe working chamber.

The secondary low pressure port may comprise one or more apertures inthe working chamber, for example, where the working chamber comprises ahollow piston, the secondary low pressure port may comprise an aperturein the hollow piston, such as an aperture in the base of the hollowpiston. The fluid working machine may be operable to bring one or morefluid conducting conduits periodically into alignment with the said oneor more apertures to thereby bring the working chamber into fluidcommunication with a manifold for a period of time, typically in phasedrelation with, and preferably phase locked to, cycles of working chambervolume. Where the fluid working machine comprises a plurality of saidworking chambers, a single fluid conducting conduit may periodicallyalign with the apertures associated with a plurality of said workingchambers in turn. Typically, the or each fluid conducting conduit isformed in a rotatable member, such as a rotatable shaft, or a rotatableeccentric or shaft having a plurality of lobes, such as a ring cam.

For example, the fluid working machine may be a piston pump, with theworking chamber having a volume defined by a cylinder and reciprocatingpiston, for example, a hollow piston. The fluid working machine may be aradial piston pump in which a cylinder has a base in sliding contactwith an eccentric attached to (typically integrated into the surface of)a rotatable crankshaft. Where the fluid working machine comprises aplurality of said working chambers defined by cylinders, each of whichhas a base in sliding contact with the same eccentric, the eccentric mayinclude one or more fluid conducting conduits adapted to periodicallybring an aperture in the base of each cylinder which is in slidingcontact with the eccentric into fluid communication with a low pressuremanifold in turn, thereby opening the secondary low pressure portassociated with each working chamber in turn in phased relation tocycles of working chamber volume to bring each working chamber into, andsubsequently out of, fluid communication with the said low pressuremanifold. The said low pressure manifold may comprise the crankshaftcase of a radial piston pump. The one or more fluid conducting conduitsmay comprise one or more peripheral slots extending around part of thecircumference of the eccentric. Thus, the or each peripheral slot mayperiodically bring the interior of pistons into fluid communication withfluid within the surrounding crankshaft case in phased relation tocycles of working chamber volume.

Alternatively, the fluid working machine may be an axial piston pump inwhich the working chamber has a volume defined by a cylinder andreciprocating piston, for example, a hollow piston, driven by and incommunication with a wobble plate, wherein the working chamber comprisesan aperture which functions as the secondary low pressure port and thewobble plate comprises one or more fluid conducting conduits adapted toperiodically bring the said aperture in the base of the cylinder intofluid communication with a low pressure manifold, thereby periodicallyopening the secondary low pressure port of the working chamber. Where aplurality of said working chambers are provided, more than one of whichhas a volume defined by a cylinder and reciprocating piston incommunication with the same wobble plate, the one or more fluidconducting conduits are preferably arranged to periodically bring theaperture in the base of each said working chamber into fluidcommunication with a low pressure manifold to thereby open the secondarylow pressure port of each said working chamber in turn. The low pressuremanifold in communication with the one or more fluid conducting conduitsmay comprise the crankshaft case of an axial piston pump. The one ormore fluid conducting conduits may comprise one or more slots in thesurface of the wobble plate arranged to periodically bring the interiorof the piston, or each of the said plurality of pistons in turn, into,and subsequently, out of, fluid communication with fluid within thesurrounding crankshaft case in phased relation to cycles of workingchamber volume.

Thus, the secondary low pressure port may comprise one or more aperturesin the working chamber which are periodically revealed, or brought intoalignment with a fluid conduit, for example, a groove inlaid into thesurface of a rotatable crankshaft. Where the working chamber comprises ahollow piston which reciprocates within a cylinder, the secondary lowpressure port may comprise an aperture in either or both of the hollowpiston, or the cylinder, which aperture is revealed, or which aperturesare aligned, during a motoring cycle, towards the end of the expansionstroke to release pressurised fluid from the working chamber, reducingthe pressure within the working chamber, and thereby facilitating theopening of the low pressure valve.

Preferably, the pressure differential between the working chamber andthe low pressure manifold into which the secondary low pressure portreleases pressurised fluid exceeds the pressure differential againstwhich the primary low pressure valve can open by a factor of at least10, and typically at least 100 or at least 1,000.

The fluid working machine may be a motor, in which case it may beoperable to carry out only motoring cycles. However, the fluid workingmachine may be operable to function as either a motor or a pump indifferent operating modes, in which case it will only carry out motoringcycles in circumstances where it is operating as a motor.

The fluid working machine typically comprises a plurality of saidworking chambers. Pressurised fluid may be released from individual saidworking chambers, or individual groups of said working chambers, atdifferent times within cycles of the volume of the respective workingchambers, for example, individual said working chambers, or individualgroups of said working chambers, may release pressurised fluid by way ofa secondary low pressure port at different times in cycles of the volumeof the respective working chambers. Thus, individual working chambers,or individual groups of working chambers, may be optimised for differentpurposes.

The fluid working machine may also comprise one or more working chamberswhich are not operable to release pressurised fluid from the workingchamber prior to the opening of the primary low pressure valve.

The fluid working machine may comprise a rotatable crankshaft having aplurality of working chambers arranged either individually, or ingroups, at axially spaced apart locations along the length of therotatable crankshaft, each axially spaced apart location having aperipheral slot in the rotatable crankshaft through which pressurisedfluid can be released from the respective working chambers, wherein atleast two peripheral slots are located at different angles around theaxis of the crankshaft so that pressurised fluid cannot be retainedsimultaneously within all of the working chambers located on one side ofthe crankshaft, thereby reducing the maximum potential resultant forceon the crankshaft. In this case, at least two peripheral slots aretypically located on separate axially spaced eccentric cams, and it maybe that the at least two said axially spaced eccentric cams are locatedat different angles around the axis of the crankshaft, with therespective peripheral slots each being located at a similar orientationrelative to the eccentric cam on which they are located.

Typically, pressurised fluid is released from the working chamber priorto the opening of a primary low pressure valve, for example by way of asaid secondary port, during consecutive cycles of working chambervolume. Although pressurised fluid may be released from the workingchamber prior to the first time that the primary low pressure valve isopened following starting of the machine, pressurised fluid is typically(additionally or alternatively) released from the working chamber duringcycles of working chamber volume after the first cycle of workingchamber volume following start of the machine. Pressurised fluid may bereleased from the working chamber prior to the opening of a primary lowpressure valve during consecutive cycles of working chamber volume. Insome embodiments, pressurised fluid is released from the working chamberprior to the opening of a primary low pressure valve during each cycleof working chamber volume.

By determining the net displacement of fluid by the working chamber on acycle by cycle basis, we refer to determining the net displacement offluid by the working chamber, during individual cycles of workingchamber volume, from amongst a plurality of possible net displacementsof fluid (which may be discrete net displacements and/or selected from acontinuous range of net displacements). In order to determine the netdisplacement of fluid by the working chamber, the controller mayactively control a plurality of electronically controllable valves.

The fluid working machine may comprise a plurality of said workingchambers. In this case, the controller may be operable to activelycontrol a plurality of electronically controllable valves, comprising atleast the primary low pressure valve associated with each of theplurality of said working chambers, in phased relationship to cycles ofworking chamber volume, to determine the net displacement of each of thesaid plurality of working chambers on a cycle by cycle basis. Typically,this determines the net throughput of fluid through the fluid workingmachine as a whole. The controller may be operable to determine the netdisplacement of fluid by individual working chambers, or groups ofworking chambers, during individual cycles of working chamber volume.

By “actively control” we refer to enabling the controller to affect thestate of an electronically controllable valve, in at least somecircumstances, by a control mechanism which consumes power and is notexclusively a passive response, for example, the opening or closing of avalve responsive solely to the pressure difference across a valve.Related terms such as “active control” should be construed accordingly.Nevertheless, the primary low pressure valve, and one or more otherelectronically controllable valves, where present, are preferably alsooperable to open or close by passive means. The primary low pressurevalve typically opens passively due to the drop in pressure within theworking chamber, such as during an intake stroke. For example, theprimary low pressure valve, or one or more other electronicallycontrollable valves, where present, may, during at least some cycles,open passively due to a pressure difference and be selectively closableunder the active control of the controller during a portion of thecycle.

By “actively control” (and related terms such as “active control”) weinclude the possibilities that the controller is operable to selectivelycause an electronically controllable valve to do one or more of open,close, remain open and/or remain closed. The controller may only be ableto affect the state of an electronically controllable valve during aportion of a working cycle. For example, the controller may be unable toopen the primary low pressure valve against a pressure difference duringthe majority of a working cycle when pressure within the working chamberis substantial. Typically, the controller actively controls theelectronically controllable primary low pressure valve, and one or moreother electronically controllable valves where present, by transmittinga control signal either directly to an electronically controllable valveor to an electronically controllable valve driver, such as asemiconductor switch. By transmitting a control signal, we includetransmitting a signal which denotes the intended state of anelectronically controllable valve (e.g. open or closed) or a pulse whichdenotes that the state of an electronically controllable valve should bechanged (e.g. that the valve should be opened or closed), or a pulsewhich denotes that the state of an electronically controllable valveshould be maintained. The controller may transmit a signal on acontinuous basis and stop or change the signal to cause a change in thestate of an electronically controllable valve, for example, theelectronically controllable primary low pressure valve, or one or moreother electronically controllable valves where present, may comprise anormally closed solenoid opened valve which is held open by provision ofan electric current and actively closed by switching off the current.

By “in phased relationship to cycles of working chamber volume” we meanthat the timing of active control by the controller of the primary lowpressure valve, and one or more other electronically controllablevalves, where present, is determined with reference to the phase of thevolume cycles of the working chamber. Accordingly, the fluid workingmachine typically comprises working chamber phase determining means,such as a position sensor. For example, where the cycles of workingchamber volume are mechanically linked to the rotation of a shaft, thefluid working machine preferably comprises a shaft position sensor, andoptionally a shaft speed sensor, and the controller is operable toreceive a shaft position signal from the shaft position sensor, andoptionally a shaft speed signal from a said shaft speed sensor. Inembodiments which comprise a plurality of working chambers, with a phasedifference between the volume cycles of different working chambers, thecontroller will typically be operable to determine the phase ofindividual working chambers.

In some embodiments, the working chamber may comprise a secondary lowpressure port (which is typically the said secondary low pressure port)which is openable and closable in phased relationship to the cycles ofworking chamber volume to connect the working chamber to a low pressuremanifold, to enable fluid to flow into or out of the working chamberconcurrently through both the primary low pressure valve and thesecondary low pressure port, during a portion of at least some cycles ofworking chamber volume.

In this way, the primary low pressure valve and secondary low pressureport work together to supply fluid into or out of the working chamber,from at least one low pressure manifold, during a portion of at leastsome cycles of working chamber volume. As a result, the fill or exhaustcharacteristics of the working chamber are better than would be the caseif the working chamber could be brought into fluid connection with oneor more low pressure manifolds only by way of the primary low pressurevalve. For example, the force acting against the expansion orcontraction of the working chamber, due to the pressure differencebetween the working chamber and the or each low pressure manifold, maybe reduced. Where the fluid is a liquid, the improved flowcharacteristics with the secondary low pressure port can eliminatecavitation while using an electronically controllable primary lowpressure valve that would otherwise have had a too small cross-sectionalarea. This may have the effect of reducing noise and/or improving theefficiency of the fluid working machine and/or increasing the operatinglife of the machine. The provision of a secondary flow path for fluidduring an expansion stroke can particularly improve the performance ofthe pump at start-up, or in cold conditions, when the hydraulic fluid isat a relatively low temperature and so has a relatively high viscosity.

Preferably, the secondary low pressure port is closed for at least partof each cycle of working chamber volume. Preferably, the primary lowpressure valve and the secondary low pressure port are closedconcurrently only during selected cycles of working chamber volume whichare determined by the controller. For example, the primary low pressurevalve may remain open throughout selected cycles of working chambervolume where determined by the controller. Preferably, the primary lowpressure valve and the secondary low pressure port are closedconcurrently between instances when the primary low pressure valve isopen. Typically, at least under some operating conditions, the primarylow pressure valve and the secondary low pressure port are closedconcurrently between consecutive periods where the primary low pressurevalve and the secondary low pressure port are open concurrently.

The at least one working chamber may have a commutator associatedtherewith to alternately attach the electronically controllable primarylow pressure valve to (i) the said low pressure manifold and (ii) a highpressure manifold, for example as disclosed in EP 1 738 077). However,the working chamber typically comprises a high pressure valve to controlthe connection of the working chamber to a high pressure manifold. Thehigh pressure valve may comprise a pressure operated check valve (e.g.in the case of a pump) or a further electronically controllable valve(e.g. in the case of a motor, or a fluid working machine operable tofunction either as a pump or a motor), which is preferably under thecontrol of the controller.

Preferably, the controller is operable, in respect of at least somecycles of working chamber volume in which both the primary low pressurevalve and the secondary low pressure port are open concurrently, tocause the primary low pressure valve to close under the active controlof the controller, to bring the working chamber out of communicationwith the or each said low pressure manifold, a period of time after thesecondary low pressure port closes. In these circumstances, thesecondary low pressure port is already closed when the controller maycause the primary low pressure valve to close to bring the workingchamber out of communication with the or each said low pressuremanifold, and so the end of a period during which the working chamber isin fluid communication with one, or optionally two or more, low pressuremanifolds, remains under the control of the controller.

The controller typically selects the net displacement of fluid throughthe working chamber on a cycle by cycle basis, for example, by selectingthe timing of the closure of the primary low pressure valve relative tothe phase of cycles of working chamber volume or, for example, byoptionally selecting an idle cycle of the working chamber in which thereis no net displacement of fluid through the working chamber, perhaps byholding the primary low pressure valve open throughout a cycle (e.g. asdisclosed in EP 0 361 927) or keeping the working chamber out of fluidcommunication with any low pressure manifold throughout a cycle (e.g. asdisclosed in WO 2007/088380). Furthermore, the controller can moreprecisely define the end of the period during which the working chamberis in fluid communication with one, or optionally two or more, lowpressure manifolds, than would be the case using a non-electronicallycontrollable valve. Typically, working chamber volume continues to varycyclically during idle cycles in which there is no net displacement offluid through the working chamber.

Accordingly, in some embodiments, the primary low pressure valve doesnot require as large a flow path cross-section as would be the case ifthe secondary low pressure port was not provided. This may allow anelectronically controllable valve with a smaller flow path cross-sectionto be employed than would otherwise be the case to obtain desiredperformance characteristics. Accordingly, the primary low pressure valvemay be selected with increased emphasis on its performance in definingthe end of the period during which a working chamber is in fluidcommunication with one, or optionally two or more, low pressuremanifolds, for example, because of its speed of closing, its ability toopen against a pressure gradient, its power consumption, or itsreliability, than would be the case if the flow path cross-section ofthe primary low pressure valve was a higher priority.

The primary low pressure valve and secondary low pressure port may eachbe openable to bring the working chamber into and out of fluidcommunication with the same low pressure manifold. Alternatively, theprimary low pressure valve and secondary low pressure port may each beopenable to bring the working chamber into and out of fluidcommunication with a different low pressure manifold. In this case, thetwo low pressure manifolds would typically have similar pressures.

It may be that the primary low pressure valve and the secondary lowpressure port are only open concurrently during an expansion stroke ofthe working chamber, for example, where the fluid working machine isoperating as a pump. The secondary low pressure port may be openableonly during an expansion stroke of the working chamber, but the primarylow pressure valve may be optionally closed under the active control ofthe controller within or just before the beginning of the contractionstroke (bottom dead centre in a piston machine) and openable at the endof the contraction stroke (top dead centre in a piston machine) of theworking chamber.

It may be that the primary low pressure valve and the secondary lowpressure port are only open concurrently during a contraction stroke ofthe working chamber, for example, in the case of a fluid working machineoperating as a motor, such as a fluid working machine in which the highpressure valve comprises an electronically controllable valve under theactive control of the controller. The secondary low pressure port may beopenable only during a contraction stroke of the working chamber, butthe electronically controllable low pressure valve may be optionallyclosed under the active control of the controller before the end of thecontraction stroke (top dead centre in a piston machine) and openable ator after the end of the contraction stroke (top dead centre in a pistonmachine).

Preferably, the primary low pressure valve and the secondary lowpressure port are both open in use, during at least some cycles ofworking chamber volume, at the point in an expansion or contractionstroke, as appropriate, where the rate of change of the volume of theworking chamber is greatest, as this is the time when the greatest rateof fluid intake or discharge respectively is required. Indeed, as thepressure difference across the primary low pressure valve isproportional to the square of the rate of fluid flow through the primarylow pressure valve, it may be sufficient for the primary low pressurevalve and the secondary low pressure port to both be open in use duringa limited portion of an expansion or contraction stroke, as appropriate.Said limited portion of an expansion or contraction stroke is preferablyless than 50%, of the duration of an expansion or contraction stroke, asappropriate, including the point in an expansion or contraction stroke,as appropriate, where the rate of change of the volume of the workingchamber is greatest.

The period of time during which both the secondary low pressure port andthe primary low pressure valve are open concurrently during selectedcycles is preferably less than 90%, and preferably more than 30%, of theduration of a contraction stroke or expansion stroke, as appropriate.This allows scope for variation in the period of time which elapsesbetween closure of the secondary low pressure port and closure of theprimary low pressure valve from cycle to cycle, to select different netdisplacements of fluid during individual cycles of working chambervolume whilst enabling the secondary low pressure port to supply orreceive additional fluid for a significant portion of the contractionstroke or expansion stroke.

It may be that the primary low pressure valve opens after the secondarylow pressure port during at least some cycles of working chamber volume.It may be that the primary low pressure valve opens before the secondarylow pressure port during at least some cycles of working chamber volume.In some embodiments, the controller is operable to determine whether theprimary low pressure valve opens before or after the secondary lowpressure port on a cycle by cycle basis.

Preferably, whichever of the primary low pressure valve and thesecondary low pressure port opens first during the said some cycles ofworking chamber volume opens at a time during the volume cycle of theworking chamber when the pressure difference between the working chamberand the low pressure manifold is minimal, for example less than 5% ofthe maximum design pressure of the working chamber.

The opening and/or closing of the secondary low pressure port may, ormay not, be controlled by the controller. The secondary low pressureport may be openable passively, for example, responsive to the pressurein the working chamber being at least a predetermined amount below thepressure in the respective low pressure manifold. Accordingly, thesecondary low pressure port may be a pressure operated valve.

In some embodiments, the secondary low pressure port is openable orclosable by a secondary electronically controllable valve, the openingor closing or both opening and closing of which is under the activecontrol of the controller, to bring the working chamber into or out offluid communication with a low pressure manifold by way of the secondarylow pressure port. The secondary low pressure port may be openable andclosable by a secondary electronically controllable valve which openspassively in use, in response to the pressure in the working chamberbeing below the pressure in the low pressure manifold. The secondary lowpressure port may be openable or closable by a secondary electronicallycontrollable valve which closes passively in use, in response to thepressure in the working chamber being above the pressure in therespective low pressure manifold.

Where the secondary low pressure port is openable or closable by meansof a secondary electronically controllable low pressure valve, theprimary low pressure valve and the secondary electronically controllablelow pressure valve may be selected to each have operatingcharacteristics which are better suited to the roles of last closing andfirst opening the connection between the working chamber and the or eachlow pressure manifold, respectively.

The secondary low pressure port may be openable other than by anelectronically controllable valve. For example, the secondary lowpressure port may be normally-closed but openable responsive to thepressure within the working chamber being a predetermined amount lessthan the pressure in the low pressure manifold communicating with thesecondary low pressure port. Thus, the secondary low pressure port maycomprise a normally-closed pressure-openable check valve.

The phase of the opening and closing of the secondary low pressure portmay be invariable relative to cycles of working chamber volume, that isto say, the opening and closing of the secondary low pressure port maybe phase locked. In the case of a fluid working machine which isoperable to function as either a pump or a motor in different operatingmodes, the opening and closing of the secondary low pressure port ispreferably not phase locked. This is because the secondary low pressureport is typically openable during the expansion stroke for a pumpingcycle and the contraction stroke for a motoring cycle, but not both.

Where the opening and closing of each secondary low pressure port isphase locked to the expansion and contraction cycle of the workingchamber, each secondary low pressure port may be opened and closed by amechanical arrangement operatively linked to the expansion andcontraction cycle of the working chamber.

The working chamber is preferably elongate at its maximum extent and theprimary low pressure valve and secondary low pressure port may beprovided spaced apart along the length of the working chamber, forexample, at or proximate to opposite ends of the working chamber. By“spaced apart along the length” we mean that a vector extending from theprimary low pressure valve to the secondary low pressure port has acomponent parallel to the length of the working chamber and do not meanto imply a limitation that the said vector is necessarily parallel tothe axis of the working chamber.

By providing paths for fluid to enter the working chamber at twodifferent locations which are spaced apart along the length of theworking chamber, the flow characteristics of fluid flowing into or outof the working chamber are better than would be the case if the primarylow pressure valve and the secondary low pressure port were adjacent.Where the working chamber is elongate whilst at maximum extent, theprimary low pressure valve and the secondary low pressure port arepreferably provided at opposite ends of the working chamber to maximisethis effect.

Where the working chamber is a piston-cylinder having a generally fixedend and a moving end (for example, in the case of a radial or axialpiston machine), the primary low pressure valve is preferably providedat the fixed end of the cylinder, to minimise movement of the primarylow pressure valve. The primary low pressure valve may be coaxial withthe cylinder or extend radially from the cylinder at the fixed end ofthe cylinder. The high pressure valve is typically also provided at thefixed end of the cylinder, typically either coaxially with or extendingradially from the low pressure valve. In these arrangements, thesecondary low pressure port is preferably provided at the opposite endof the cylinder. This has the advantage of causing an exchange of fluidin all parts of the cylinder on each cycle, reducing hot spots in thefluid around the base of the cylinder. For example, the secondary lowpressure port may be coaxial with or extend radially from the cylinder,at the moving end of the cylinder.

The controller is operable to control the opening and/or closing of theprimary low pressure valve. Where the high pressure valve comprises anelectronically controllable valve, the controller is preferably operableto control the opening and/or closing of the said electronicallycontrollable valve. Where the secondary low pressure port is openableand/or closable by a secondary electronically controllable low pressurevalve, the controller is preferably operable to control the openingand/or closing of the secondary electronically controllable low pressurevalve.

The controller is preferably operable to control the opening and/orclosing of the at least one electronically controllable valve(comprising at least the primary low pressure valve) on a cycle by cyclebasis by either, or preferably both, of determining whether or not toopen and/or close a specific electronically controllable valve during aspecific cycle, and determining the phase of the opening and/or closingof a specific electronically controllable valve relative to a cycle ofthe volume of the working chamber. By controlling the opening and/orclosing of the at least one electronically controllable valve we includethe possibility of holding a valve open or closed.

Typically, by controlling the opening and/or closing phase of the atleast one electronically controllable valve (comprising at least theprimary low pressure valve) on a cycle by cycle basis, the controller isoperable to cause the working chamber to displace a volume of fluidselected from a plurality of different selectable volumes, on a cycle bycycle basis. Typically, the plurality of different selectable volumesincludes the maximum volume displaceable by an individual workingchamber, and no net displacement. No net displacement may be achieved byan idle cycle in which the electronically controllable low pressurevalve remains open throughout a cycle of working chamber volume or bysealing the working chamber throughout a cycle of working chambervolume, for example as described in WO 2007/088380. By displacement werefer to the net movement of fluid from the or each low pressuremanifold to the (or each) high pressure manifold, or vice versa, and donot refer to any net movement of fluid between low pressure manifolds,or high pressure manifolds, which may occur. The plurality of differentselectable volumes preferably also includes at least one volume, andpreferably a plurality of volumes (for example, a continuous range ofvolumes) between no net displacement and the maximum volume displaceableby the working chamber. However, where a plurality of working chambersare provided, the controller may also control groups of working chambersin this manner. The controller typically balances the time averaged netthroughput of fluid of one or more working chambers against a receiveddemand signal which may be constant or variable. The fluid workingmachine is typically used in combination with high and/or low pressureaccumulators in communication with the high and/or low pressuremanifolds respectively to smooth the pressure or flow of the inputand/or output fluid.

The one or more electronically controllable valves (including theelectronically controllable primary low pressure valve, and the highpressure valve and/or the secondary electronically controllable valvewhere provided) are typically face-sealing valves. The one or moreelectronically controllable valves (including the electronicallycontrollable primary low pressure valve, and the high pressure valveand/or the secondary electronically controllable valve where provided)are typically poppet valves. The one or more electronically controllablevalves (including the electronically controllable primary low pressurevalve, and the electronically controllable high pressure valve and/orthe secondary electronically controllable valve where provided) may beelectromagnetically actuated poppet valves. The one or moreelectronically controllable valves (including the electronicallycontrollable primary low pressure valve, and the electronicallycontrollable high pressure valve and/or the secondary electronicallycontrollable valve where provided) may be solenoid operated poppetvalves.

The primary low pressure valve is typically inward opening, toward theworking chamber. The high pressure valve is typically outward opening,away from the working chamber.

The fluid working machine may be a pump. The fluid working machine maybe a motor. The fluid working machine may be operable to function aseither a pump or a motor in alternative operating modes. The fluidworking machine may further comprise one or more manifolds incommunication with the primary low pressure valve, secondary lowpressure port and/or high pressure valve.

In embodiments in which the fluid working machine comprises a pluralityof said working chambers, the optional and preferred features discussedherein typically apply to each said working chamber and the primary lowpressure valve, secondary low pressure port and, where relevant, highpressure valve associated with each said working chamber, asappropriate. Typically, the or each low and high pressure manifold is incommunication with more than one (for example, each) of the plurality ofsaid working chambers.

According to a second aspect of the present invention there is provideda method of operating a fluid working machine working chamber ofcyclically varying volume, during a motoring cycle of the workingchamber, comprising opening an electronically controllable primary lowpressure valve, in phased relation to cycles of working chamber volume,to bring the working chamber into fluid communication with a lowpressure manifold under the active control of a controller on a cycle bycycle basis, characterised in that the method further comprisesreleasing pressure within the working chamber prior to the opening ofthe primary low pressure valve, during the expansion stroke of a saidmotoring cycle.

The resulting release of pressurised fluid preferably facilitates theopening of the primary low pressure valve. Preferably, pressure isreleased within the working chamber prior to the opening of the primarylow pressure valve by opening a secondary low pressure port, throughwhich fluid can be released from the working chamber.

Preferably also, the secondary low pressure port is opened by amechanical arrangement which is operatively linked to cycles of workingchamber volume. Typically, the fluid working machine comprises arotatable shaft, and the opening of the secondary low pressure port ismechanically linked to the rotatable shaft.

In some embodiments, the method comprises the step of opening a or thesaid secondary low pressure port, in phased relation to cycles ofworking chamber volume, to bring the working chamber into fluidcommunication with a low pressure manifold by a second path, such that,during a portion of at least some cycles of working chamber volume, theprimary low pressure valve and secondary low pressure port are openconcurrently such that fluid flows into or out of the working chamber,as appropriate, through both the primary low pressure valve and thesecondary low pressure port.

Preferably, during at least some cycles of working chamber volume inwhich both the said primary low pressure valve and the said secondarylow pressure port are open concurrently, the controller is operable toclose the primary low pressure valve a period of time after thesecondary low pressure port closes.

The invention also extends in a third aspect to program code which, whenexecuted on a fluid working machine controller, causes the fluid workingmachine to function as a fluid working machine according to the firstaspect of the invention or to carry out a method according to the secondaspect of the invention.

The program code may take the form of source code, object code, a codeintermediate source, such as in partially compiled form, or any otherform suitable for use in the implementation of the methods of theinvention. The program code may be stored on or in a carrier, which istypically a computer readable carrier such as a ROM, for example a CDROM or a semiconductor ROM, or a magnetic recording medium, for examplea floppy disc or hard disc. Furthermore, the carrier may be atransmissible carrier such as an electrical or optical signal which maybe conveyed via electrical or optical cable or by radio or other means.When a program is embodied in a signal which may be conveyed directly bycable, the carrier may be constituted by such cable or other device ormeans.

DESCRIPTION OF THE DRAWINGS

An example embodiment of the present invention will now be illustratedwith reference to the following Figures in which:

FIG. 1 is a schematic diagram of an individual working chamber of afluid working machine;

FIG. 2 is a timing diagram illustrating the status of the primary lowpressure valve, the secondary low pressure port, and the high pressurevalve, as well as the pressure within a working chamber during a pumpingcycle;

FIG. 3 is a schematic diagram of fluid flow into a working chamber of ahydraulic radial piston pump, during an expansion stroke;

FIG. 4 is a schematic diagram of fluid flow out of a working chamber ofthe hydraulic radial piston pump of FIG. 3, during a contraction stroke;

FIG. 5 is a timing diagram illustrating the status of the primary lowpressure valve, the secondary low pressure port, and the high pressurevalve, as well as the pressure within a working chamber during amotoring cycle;

FIG. 6 is a timing diagram for a hydraulic motor, or hydraulicpump/motor, having a depressurising port, illustrating the status of theprimary low pressure valve, depressurising port, and a high pressurevalve, as well as the pressure within a working chamber, and the crankshaft torque, during a motoring cycle;

FIG. 7 is a schematic diagram of fluid flow out of a working chamber ofa hydraulic motor, or hydraulic pump/motor having a depressurising port;

FIG. 8 is a schematic diagram of fluid flow out of a working chamber ofan alternative embodiment of a hydraulic motor, or a hydraulicpump/motor, with a depressurising port;

FIG. 9 is a schematic diagram of fluid flow out of the working chamberof a further example of a hydraulic motor, or hydraulic pump/motor witha depressurising port; and

FIG. 10 is a schematic diagram showing the reduction in resultant forceson a crankshaft from the release of pressurised fluid from two banks ofpistons.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS Example One

In a first example, a fluid working machine in the form of a hydraulicpump includes a plurality of working chambers. FIG. 1 illustrates anindividual working chamber 2 which has a volume defined by the interiorsurface of a cylinder 4 and a piston 6 which is driven from a crankshaft8 by a crank mechanism 9 and which reciprocates within the cylinder tocyclically vary the volume of the working chamber. A shaft position andspeed sensor 10 determines the instantaneous angular position and speedof rotation of the shaft, and transmits shaft position and speed signalsto a controller 12, which enables the controller to determine theinstantaneous phase of the cycles of each individual working chamber.The controller is typically a microprocessor or microcontroller whichexecutes a stored program in use.

The working chamber comprises a primary low pressure valve in the formof an electronically actuatable face-sealing poppet valve 14, whichfaces inwards toward the working chamber and is operable to selectivelyseal off a channel extending from the working chamber to a first lowpressure manifold 16, which functions generally as a net source of fluidin use. The primary low pressure valve is a normally open solenoidclosed valve which opens passively when the pressure within the workingchamber is less than the pressure within the first low pressuremanifold, during an intake stroke, to bring the working chamber intofluid communication with the first low pressure manifold, but isselectively closable under the active control of the controller to bringthe working chamber out of fluid communication with the first lowpressure manifold. One skilled in the art will appreciate thatalternative electronically controllable valves may be employed, such asnormally closed solenoid opened valves.

The working chamber further comprises a high pressure valve 18 in theform of a pressure actuated delivery valve. The high pressure valvefaces outwards from the working chamber and is operable to seal off achannel extending from the working chamber to a high pressure manifold20, which functions as a net sink of fluid in use.

The high pressure valve functions as a normally-closedpressuring-opening check valve which opens passively when the pressurewithin the working chamber exceeds the pressure within the high pressuremanifold.

A secondary low pressure port 22 is openable and closable by means of asecondary low pressure valve 24 which, when open, brings the workingchamber into fluid communication with a second low pressure manifold 26,which also functions as a net source of fluid in use. In this example,the primary low pressure valve and the secondary low pressure port areconnected to two distinct low pressure manifolds of similar pressure.However, they may alternatively be connected to the same low pressuremanifold. The opening and closing of the secondary low pressure port maybe phase locked to the working cycle of the working chamber, forexample, by virtue of a mechanical linkage 28 between the crankshaft andthe secondary low pressure valve. Alternatively, the opening or closingof the secondary low pressure valve may be actively controlled by thecontroller, by virtue of an electronic connection 30. Alternatively, thesecondary low pressure valve may be a normally-closed pressure-openablecheck valve which opens responsive to a drop in the pressure of theworking chamber relative to the second low pressure manifold in whichcase neither the mechanical linkage nor the electronic connection needto be present.

FIG. 2 is a timing diagram illustrating the status of the primary lowpressure valve, the secondary low pressure port, and the high pressurevalve, as well as the pressure within the working chamber during apumping cycle. The primary low pressure valve opens at or around topdead centre due to the pressure difference between the first lowpressure manifold and the working chamber which allows fluid to flowinto the working chamber from the first low pressure manifold to beginan intake stroke. The increasing velocity of fluid past the primaryvalve causes the working chamber pressure to fall until, for a period oftime during the intake stroke, the secondary low pressure valve opens.Opening of the secondary low pressure port may be mechanically phaselocked to the position of the crankshaft and occur a period of timeafter the opening of the primary low pressure valve. Alternatively, theopening of the secondary low pressure valve may be caused by theincreasing pressure difference between the low pressure manifold and theworking chamber. The secondary low pressure port is open at the point inthe pumping cycle when the rate of change of working cylinder volume isgreatest and the additional fluid flow is of greatest benefit.

Once the secondary low pressure valve has opened, hydraulic fluid entersthe working chamber from the low pressure manifold via both the primarylow pressure valve and the secondary low pressure port. After a periodof time, the secondary low pressure valve closes so that fluid onceagain enters the working chamber from the low pressure manifold onlythrough the primary low pressure valve.

The controller determines the phase of the working chamber pumping cycleusing the received shaft position and speed signals and, at or aroundbottom dead centre, makes a decision as to whether to select a pumpingcycle or an idle cycle. In the example illustrated in FIG. 2, thecontroller selects a pumping cycle and sends a signal causing theprimary low pressure valve to close. The primary low pressure valvecloses a period of time after the closure of the secondary low pressureport. Once the primary low pressure valve closes, the working chamber isisolated from the low pressure manifolds, the pressure in the workingchamber increases and the high pressure valve opens to receive a definedvolume of fluid into the high pressure manifold. During other cycles,the controller may alternatively cause the primary low pressure valve toremain open so that low pressure fluid received from both low pressuremanifolds is vented back to the first low pressure manifold with no netdisplacement of fluid from the low pressure manifolds to the highpressure manifolds.

By providing a secondary low pressure port, the flow characteristics ofthe hydraulic fluid entering the working chamber during an intake strokeare better than would be the case if only the primary low pressure valvewas provided. For example, less cavitation occurs and less drag isexerted to resist expansion of the working chamber than would otherwisebe the case. However, because the opening and closing of the secondarylow pressure port is phased away from the opening and closing of theprimary low pressure valve, the electronically controllable primary lowpressure valve controls the timing of the communication between theworking chamber and the first low pressure manifold to start and finishthe intake stroke. Thus, the primary low pressure valve may have asmaller fluid flow cross-section than would be the case if all of thefluid entered the working chamber through the primary low pressurevalve.

Importantly, as well as determining whether or not to close or hold openthe primary low pressure valve on a cycle by cycle basis, the controlleris operable to vary the precise phasing of the closure of the primarylow pressure valve with respect to the varying working chamber volume todetermine the net displacement of fluid from the low pressure manifoldsto the high pressure manifold during a pumping cycle. As describedabove, by keeping the primary low pressure valve open throughout a cyclean idle stroke can occur in which, although fluid flows into the workingchamber from the low pressure manifolds and flows out to the first lowpressure manifold there is no net displacement from the low pressuremanifolds to the high pressure manifold. (There may be net displacementfrom the second low pressure manifold to the first low pressuremanifold, but this is not considered to be net displacement by thepump). A partial stroke which displaces a volume of fluid equal to aproportion (usually a relatively small proportion) of the capacity ofthe working chamber may be implemented by delaying closure of theprimary low pressure valve and opening of the high pressure valve untiljust before top dead centre, and the precise volume which is displacedmay be selected by the precise timing of these events. The precisetiming of the opening and/or closing of the primary low pressure valveand the high pressure valve may also be varied in specificcircumstances, such as start-up, operation while still relatively cold,and shut down of the device. Further details of these timing options aredisclosed in EP 0 361 927, EP 0 494 236 and EP 1 537 333, the contentsof which are incorporated herein by virtue of this reference.

Fluid discharged through the high pressure manifold is typicallydelivered to a pressure accumulator to smooth the output pressure andthe time averaged throughput is varied by the controller on the basis ofa demand signal received by the controller in the manner of the priorart.

Example Two

In a second example, the fluid working machine is operable to functionas either a motor or a pump. The structure of the second example fluidworking machine also corresponds to the structure illustrated in FIG. 1.In this embodiment, the primary low pressure valve functions as a netsource of fluid or a net sink in pumping or motoring mode respectively.The secondary low pressure port also functions as either a net source offluid or net sink respectively, and the high pressure valve functions aseither a net sink of fluid or net source respectively. A single lowpressure manifold functions as either a net source of fluid, in pumpingmode, or as a sink of fluid, in motoring mode, and the high pressuremanifold functions as either a sink of fluid, in pumping mode, or as asource of fluid, in motoring mode. During idle strokes in which aworking chamber is kept in fluid communication with the low pressuremanifold, neither manifold functions as a net source or sink of fluid.

As with the first example, the primary low pressure port is an inwardfacing electronically controllable poppet valve. However, in thisexample, the secondary low pressure port and the high pressure valvealso comprise electronically actuatable poppet valves which face inwardsand outwards respectively and which are actively controllable by thecontroller on a cycle by cycle basis through electronic connections 30and 32. In pumping mode, the timings of the secondary low pressure portand the high pressure valve are the same as in the first example. Inmotoring mode, fluid is received through the high pressure valve duringworking chamber expansion strokes to drive the crankshaft and outputthrough the primary low pressure valve during working chambercontraction strokes. The secondary low pressure port opens for a portionof the contraction stroke to provide an additional path for fluid to bedisplaced from the working chamber.

By using an electronically controllable valve to regulate the secondarylow pressure port, rather than a mechanical arrangement driven from thecrankshaft, the controller can open the secondary low pressure portduring expansion strokes when the fluid working machine is operating asa pump and during contraction strokes when the fluid working machine isoperating as a motor.

In an alternative implementation of this second example embodiment, thesecondary low pressure port may be closed by means of apressure-operated check valve not under the control of the controller.The pressure-operated check valve allows fluid to be received into thecylinder from the low pressure manifold on the expansion stroke when theprimary low pressure valve is open. By using a pressure-operated checkvalve to provide a second path for fluid to enter the working chamber,the working chamber is more easily able to receive fluid from the lowpressure manifold and can thus avoid cavitation. The pressure-operatedcheck valve will be closed on the contraction stroke either whenexhausting to the low pressure manifold in an idle or motor exhauststroke, and closed on the expansion stroke during a motor stroke.

Example Three

In a third example embodiment a fluid working machine in the form of ahydraulic radial piston pump uses a slotted crankshaft to provide asecondary low pressure port. FIG. 3 illustrates fluid flow through anindividual working chamber 100, defined by the interior surface of acylinder 102 and reciprocating hollow piston 104, part way through anexpansion stroke.

The working chamber has a primary low pressure valve 106, in the form ofan electronically controllable poppet valve, which is openable andclosable to bring the working chamber into and out of fluidcommunication with a first low pressure manifold 108. A high pressurevalve in the form of a pressure-operable discharge valve 110 is openableand closable to bring the working chamber into and out of fluidcommunication with a high pressure manifold 112. The base 114 of thepiston is in sliding contact with a crankshaft eccentric 116. Anaperture 118 in the base of the piston functions as a secondary lowpressure port which is open when a slot 120, which extends around aportion of periphery of the eccentric, extends across either side of thepiston wall to bring the interior of the working chamber into fluidcommunication with hydraulic fluid within the crankshaft case 122, whichfunctions as a second low pressure manifold. Accordingly, for a portionof the expansion stroke, fluid will flow into the working chamber both(i) through the primary low pressure valve and (ii) through thecrankshaft slot and the aperture in the base of the piston.

As before, the secondary low pressure port opens a period of time afterthe primary low pressure valve opens due to the pressure in the workingchamber 100 falling to a level where it is no longer held closed, andthe secondary low pressure port closes a period of time before thecontroller may optionally send a signal to cause the primary lowpressure valve to close so as initiate the pumping mode on thecontraction stroke.

FIG. 4 illustrates the fluid flow during the subsequent contractionstroke, where the primary low pressure valve and secondary low pressureport are both closed, by the electronically controllable poppet valveand the body of the crankshaft eccentric respectively, and fluid isdisplaced to the high pressure manifold through the high pressuredischarge valve. The opening and closing of the secondary low pressureport is phase locked to the cycles of working chamber volume, as definedby the location of the slot on the crankshaft eccentric. The variationin working chamber pressure during the expansion and contraction strokescorresponds to that illustrated in FIG. 2.

This arrangement has several advantages. Firstly, by supplying fluidconcurrently from either end of the elongate working chambers during thepart of the expansion stroke where the volume of the working chambers ismost rapidly increasing, fluid need not flow as quickly and so the flowcharacteristics of fluid entering the working chambers are improved.Secondly, there is not a pool of fluid at the moving end of each workingchamber which can remain in place from one cycle to the next. A freshsupply of fluid enters the aperture in the base of each piston duringeach cycle, cooling the base of each piston. Furthermore, centrifugalforces act in the same direction as net fluid flow from the crankshaftto the high pressure outlet, increasing the overall efficiency of thepump.

Example Four

The arrangement of FIGS. 3 and 4 can operate as a hydraulic radialpiston motor by the use of an active high pressure valve and by changingthe location of the slot on the crankshaft to amend the phase of theopening of the secondary low pressure port so that the secondary lowpressure port opens during the contraction stroke rather than theexpansion stroke. FIG. 5 illustrates the opening and closing of thephase-locked secondary low pressure port during the motoring cycle. Inthis case the pressure in the working chamber rises during the exhaustof fluid to the low pressure manifold through the primary low pressureport, until the opening of the phase-locked secondary low pressure portprovides an alternative flow path and reduces the working chamberpressure.

Example Five

A fifth example embodiment addresses technical problems related to theopening of a low pressure valve during a motoring cycle of a fluidworking motor, or a fluid working machine which can operate as either amotor or a pump, in different operating modes.

This embodiment corresponds to the hydraulic radial piston motor ofExample Four, except that the location of the slot on the crankshaft ispositioned so that the secondary low pressure port opens shortly beforethe end of the expansion stroke, after the high pressure valve hasclosed, is phase locked to cycles of working chamber volume.

The effect of this arrangement on the operation of the fluid workingmachine is illustrated in FIG. 6. The operation of the fluid workingmotor is conventional during the first part of the expansion stroke.Pressurised fluid is received from the high pressure manifold into theworking chamber, through an active high pressure valve. Once the highpressure valve is closed, the pressure within the working chamber beginsto decrease, however, the fluid within the working chamber remainspressurised. After the closure of the high pressure valve, but beforebottom dead centre, the slot aligns with the base of the working chamberpiston forming a secondary low pressure port. Pressurised fluid ventsfrom the interior of the working chamber into the crankshaft case viathe crankshaft slot. Accordingly, the pressure within the workingchamber drops rapidly to close to the pressure of the low pressuremanifold. The low pressure valve, which is gently biased to an openposition by a weak spring, therefore opens passively against only aminimal pressure differential. Shortly after bottom dead centre, theslot no longer aligns with the base of the piston and so the secondarylow pressure port closes. The low pressure valve may alternatively bedragged open when the pressure within the working chamber issufficiently low.

Because the slot is integral to the crankshaft, it can open despite thesubstantial pressure differential between the working chamber and thesurrounding crankshaft case. An electronically controllable low pressurevalve which could open against the substantial pressure differentialswhich occur at this point in a motoring cycle in many practicalapplications would require considerable power and/or open more slowly.Furthermore, the provision of a secondary low pressure port, or otherdepressurising means, enables the time which elapses between the closureof the high pressure valve and the opening of the low pressure valve tobe less than would otherwise be the case, allowing the high pressurevalve to close later and/or the low pressure valve to open earlier thanwould otherwise be the case and thereby minimising the amount of timethat the working chamber is not either receiving high pressure fluid orreleasing fluid to the low pressure manifold, and thereby increasing theenergy efficiency of the fluid working machine. In the exampleillustrated in FIG. 6, were it not for the release of pressurised fluidusing the secondary low pressure port, the pressure within the workingchamber would follow the path illustrated with a dashed line, in whichcase the low pressure valve would not open.

It is also envisaged that the secondary low pressure port could remainopen until at least the point in the contraction stroke where the volumeof the working chamber is most rapidly changing, to enable fluid to flowout of the working chamber to the low pressure manifold through both theprimary low pressure valve and the secondary low pressure portconcurrently.

Example Six

In further example embodiment, illustrated in FIG. 8, an aperture 123,is provided towards the radially outwards end of a piston. The portionof the piston which includes the aperture extends out of the cylinder,forming a secondary low pressure port through which hydraulic fluid canbe released to the crankshaft case, from shortly before to shortly afterbottom dead centre. In an alternative embodiment, illustrated in FIG. 9,an aperture 124 is instead provided towards the radially inwards end ofthe cylinder, forming a secondary low pressure port through whichpressurised hydraulic fluid can be released to the crankshaft case, fromshortly before to shortly after bottom dead centre. In a furtherembodiment, apertures can be provided in each of the piston and thecylinder which overlap for a period of time from shortly before toshortly after bottom dead centre.

One skilled in art will appreciate that secondary low pressure portswhich open to vent fluid from the working chamber of a fluid workingmachine, during a motoring stroke, to facilitate the opening of aprimary low pressure valve, could be implemented in numerous ways.Mechanically linking the opening and closing of the secondary lowpressure port to cycles of working chamber volume has the advantage thatthe secondary low pressure port can be opened against a substantialpressure differential.

With reference to FIG. 10, one possible implementation of the inventionis in a fluid working machine, which includes a crankshaft, with aplurality of banks of working chambers (130 a to 130 f, and 132 a to 132f) arranged at axially spaced apart locations along the crankshaft, eachbank having an eccentric cam 116. Preferably, the eccentric cams arearranged in different phases with respect to each other. In this case, aperipheral slot in each crankshaft eccentric (122 a and 122 b) can beprovided in respect of each bank of working chambers, and the peripheralslots in each crankshaft eccentric can be arranged at similarorientations with respect to the eccentric in which they lie, so that itis not possible to retain pressurised fluid simultaneously within allworking chambers on any one side of the crankshaft. Because pressurisedworking chambers apply forces orthogonal to the axis of the crankshaftto said crankshaft, this reduces the maximum potential resultant forceon the crankshaft, in a plane orthogonal to the axis of the crankshaft,reducing the net forces on the crankshaft, potentially increasingoperating lifetime, and reducing vibration.

Further variations and modifications may be made within the scope of theinvention herein disclosed.

1. A fluid working machine comprising a controller and a working chamberof cyclically varying volume, the working chamber having a high pressurevalve associated therewith to control the connection of the workingchamber to a high pressure manifold, and an electronically controllableprimary low pressure valve to control the connection of the workingchamber to a low pressure manifold, the controller being operable toactively control at least the primary low pressure valve, in phasedrelationship to cycles of working chamber volume, to determine the netdisplacement of fluid by the working chamber on a cycle by cycle basis,the fluid working machine being operable to carry out a motoring cycleunder at least some circumstances, characterised in that the fluidworking machine is adapted to release pressurised fluid from the workingchamber prior to the opening of the primary low pressure valve, during amotoring cycle.
 2. A fluid working machine according to claim 1, furthercomprising depressurisation means which are operable to releasepressurised fluid from the working chamber prior to the opening of theprimary low pressure valve, during a motoring cycle, to facilitate theopening of the primary low pressure valve.
 3. A fluid working machineaccording to claim 1, wherein the working chamber has a secondary lowpressure port associated therewith, which is openable and closable inphased relationship to the cycles of working chamber volume to releasepressurised fluid from the working chamber, prior to the opening of theprimary low pressure valve, during a motoring cycle, to reduce thepressure within the working chamber and thereby facilitate the openingof the primary low pressure valve.
 4. A fluid working machine accordingto claim 3, wherein the secondary low pressure port is openable andclosable in phased relationship to the cycles of working chamber volumeto release pressurised fluid from the working chamber, by way of amechanical arrangement operatively linked to the expansion andcontraction cycle of the working chamber.
 5. A fluid working machineaccording to claim 4, wherein the secondary low pressure port comprisesone or more apertures in the working chamber and the fluid workingmachine comprises one or more fluid conducting conduits, the fluidworking machine being operable to periodically bring the one or morefluid conducting conduits into alignment with the one or more aperturesto thereby bring the working chamber into fluid communication with a lowpressure manifold for a period of time, in phased relation to cycles ofworking chamber volume.
 6. A fluid working machine according to claim 5,wherein the fluid working machine is a radial piston motor, in which theworking chamber has a volume defined by a cylinder and reciprocatingpiston, the piston having a base in sliding contact with an eccentricattached to a rotatable crankshaft, the secondary low pressure portcomprising an aperture in the base of the cylinder, wherein theeccentric comprises one or more fluid conducting conduits adapted toperiodically bring the aperture into fluid communication with a lowpressure manifold in phased relation to cycles of working chambervolume.
 7. A fluid working machine according to claim 1, wherein thefluid working machine is a radial piston motor, in which the workingchamber has a volume defined by a cylinder and reciprocating piston,wherein either or both of the cylinder and the reciprocating pistoncomprises an aperture which is periodically unobscured in phasedrelation to cycles of working chamber volume to form a secondary lowpressure port through which pressurised fluid is released.
 8. A fluidworking machine according to claim 1, wherein the primary low pressurevalve and the secondary low pressure port are each openable to bring theworking chamber into and out of fluid communication with different lowpressure manifolds.
 9. A fluid working machine according to claim 1,wherein the pressure differential between the working chamber and thelow pressure manifold into which the secondary low pressure portreleases pressurised fluid exceeds the pressure differential againstwhich the primary low pressure valve can open by a factor of at least10.
 10. A fluid working machine according to claim 1, wherein thesecondary low pressure port remains open in use until the point in thesubsequent contraction stroke where the rate of decrease of workingchamber volume is greatest, to facilitate the flow of fluid out of theworking chamber to one or more low pressure manifolds.
 11. A method ofoperating a fluid working machine working chamber of cyclically varyingvolume, during a motoring cycle of the working chamber, comprisingopening an electronically controllable primary low pressure valve, inphased relation to cycles of working chamber volume, to bring theworking chamber into fluid communication with a low pressure manifoldunder the active control of a controller on a cycle by cycle basis,characterised in that the method further comprises releasing pressurisedfluid from the working chamber prior to the opening of the primary lowpressure valve, during the expansion stroke of a said motoring cycle.12. A method according to claim 11, wherein pressurised fluid isreleased from the working chamber by depressurising means.
 13. A methodaccording to claim 11, wherein the pressure is released within theworking chamber prior to the opening of the primary low pressure valveby opening a secondary low pressure port, through which fluid can ventfrom the working chamber.
 14. A method according to claim 13, whereinthe secondary low pressure port is opened by a mechanical arrangementwhich is operatively linked to cycles of working chamber volume. 15.Program code which, when executed on a fluid working machine controller,causes the fluid working machine to function as a fluid working machineaccording to claim
 1. 16. Program code which, when executed on a fluidworking machine controller, causes the fluid working machine to operateaccording to the method of claim 11.